Add Apple container sentinel file

[culpen90]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

Fixes #140.

This adds a read-only /run/.containerenv sentinel file for Linux containers so applications have a reliable Apple container detection signal without changing the process environment.

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

swift build --target ContainerRuntimeLinuxServer

[jglogan]

@culpen90 Thank you for the enhancement!

Have a look at the comment from Anders at the bottom of the issue: #140 (comment).

Can you implement the container sentinel using an env file instead? Ideally this change could be limited to just the RuntimeService.

[culpen90]

Thanks, updated this to use an env file sentinel instead.

The runtime now writes .containerenv with container=apple under the RuntimeService root and mounts it read-only at /run/.containerenv. The process environment is no longer modified, and the PR diff is limited to RuntimeService.swift.

Validated locally with:

swift build --target ContainerRuntimeLinuxServer